1. Field of the Invention
This invention relates to the coating of metal articles and to electrical equipment, e.g. made of copper, silver, or aluminum, such as coils, compressors, armatures, transformers, motors, and the like, with an aqueous solution of a water reducible epoxy resin that exhibits superior resistance to refrigerants, to chemical attack, and has excellent hot and cold bond strengths on helical coils.
2. The Prior Art
In the manufacture of electrical equipment for air conditioners and refrigerators where a refrigerant like Freon 22 is used, coils and their motor components are coated with a hermetic-type varnish. Varnishes of this type have a blend of an epoxy resin and a phenolic resin dissolved in a mixture of glycol ethers or esters and an aromatic hydrocarbon solvent. In the baking operation the evaporation of the aromatic hydrocarbon poses an air pollution problem.
One of the approaches to obviate this problem would be to formulate a water-borne coating which eliminates the use of aromatic hydrocarbons, and complies with EPA (Environmental Protection Agency) regulations.
Although aqueous systems having reactive carboxylic acid groups attached to epoxy esters have been proposed in the art, they have been deficient in resistance to refrigerants and bond strengths have been low. The products are hydrolytically unstable in the liquid and baked states. Emulsions of solid epoxy resins have posed rheological problems and their storage stability is only fair at best.
In Allen U.S. Pat. No. 4,094,844, Allen U.S. Pat. No. 4,098,744, Allen U.S. Pat. No. 4,119,609, and Birkmeyer German OS No. 2,809,403 (acknowledging U.S. application Ser. Nos. 775,489 and 775,490, filed Mar. 8, 1977), water-soluble coatings based on adducts of epoxy compounds with amino-substituted aromatic carboxylic acids are proposed as beverage can linings. These coatings, when applied to copper panels or coated copper wire, suffer from poor rheology and poor chemical resistance in a hermetic application. Moreover, their storage stability is less than two months when an aminoplast curative is present. The entire disclosure of the three Allen patents and the Birkmeyer German OS are hereby incorporated by reference and relied upon.